1. Field of the Invention
The invention relates to a condensing steam turbine having a live-steam inlet, an exhaust-steam outlet and at least two seals for sealing off the turbine casing in the region of a turbine shaft carrying a turbine rotor, at least one of the seals is disposed on the live-steam side and one of the seals is disposed on the exhaust-steam side.
Steam turbines are used in one type of construction as back-pressure turbines when the steam on the outlet side is to be utilized in a heat network at increased pressure. In another type of construction designated as a condensing turbine, the heat content of the steam is utilized by its complete expansion down to a vacuum relative to the atmosphere. The consequence thereof is that the outer seals of a condensing turbine must be suitable for preventing the ingress of air into the turbine casing which is filled with steam.
The cost for the sealing measures in the prior art shown in FIG. 3 and described below is exceptionally high, since the quantities of steam escaping also have to be discharged in a barrier-steam condenser and since the steam states for the barrier steam to be used have to be separately adapted to the temperatures of the hot turbine inlet and the turbine outlet.
Furthermore, it is known to use mechanical surface seals in turbomachine construction in stationary compressors and in aircraft engines. German Published, Non-Prosecuted Application DE 35 33 829 A1 discloses a seal construction that is especially suitable for the balancing piston of a steam turbine. The use of that seal instead of an inner balancing-piston labyrinth in a steam turbine is described. In that case a lifting device is intended to prevent damage from occurring in the region of the seal during the warm-up and condensing phase of the steam. The lifting device is also suitable for protecting the stoppage and turning operation of a turbine.